Systems exist for measuring the height or altitude of an aircraft. An airborne radar altimeter can be used to measure geometric height (i.e., distance between the aircraft and ground). An airborne barometric altimeter can be used to provide a measure of barometric altitude (i.e., an estimate of altitude above mean sea level based on comparing measured barometric pressure to a standard atmosphere value).
However, even if such measurements are accurately performed, the radar altimeter provides readings during level flight which will vary widely depending on whether the aircraft is passing over a valley or a mountain. While a ground-based precision radar might be used to accurately determine height of a passing aircraft above a defined datum level (based on radar measurements adjusted for the elevation of the radar site), such radars are expensive and not available in many geographical areas. Similarly, even if a barometric altimeter accurately measures barometric pressure and converts the pressure reading to a corresponding altitude, such conversion merely provides an altitude value from a pressure/altitude chart or table representing standard atmosphere data, such as provided by the International Civil Aviation Organization ("ICAO"). FIG. 1 is an example of such a chart, in which P is a scale of pressure in millibars, A is a scale of altitude in thousands of feet and S is a standard ICAO pressure/altitude profile for a temperature lapse rate of two degrees centigrade per one thousand feet and a temperature of 15 degrees centigrade at mean sea level (MSL). A problem in using such charts is that an aircraft does not fly in a standard atmosphere, but in the real atmosphere which is subject to temporal and spatial weather differences affecting the barometric pressure measured at any aircraft altitude. As a result, since there will virtually always be a discrepancy between the actual pressure as measured at the aircraft location and the standard pressure for the aircraft elevation, there will virtually always be a discrepancy in a barometric altimeter reading.
Thus, even with all equipment accurately calibrated, a radar altimeter can provide relative height above the earth's surface, but when operating over land it cannot reliably measure height above a reference datum like MSL. Also, a barometric altimeter measures altitude based on sensing of barometric pressure, but pressure varies in an unpredictable manner for a given geometric height and does not provide a repeatable reference relative to a datum like MSL. The preceding discussion does not address calibration difficulties and resulting error. For example, a barometric altimeter on an aircraft must be arranged to attempt to measure static pressure in a moving air stream subject to variations in aircraft speed and altitude, temperature and humidity, and subject to possible changes in aircraft configuration, damaged or blocked sensors, and barometer decalibration over time with no adequate means of recalibration.
A typical practical problem is the requirement for continuous data acquisition to permit evaluation of changing weather conditions for control of civilian air traffic, weather forecasting and a variety of other civilian, commercial and military applications. For weather forecasting, as well as for air traffic control, it will be apparent that there is a continuing need for current data on actual atmospheric conditions at different geometric heights on a local, national and global basis. While many types of relevant data can be gathered, and sophisticated analysis and plotting of data can be provided, one particular need has continued unanswered. That is the need to know, for different geometric heights at different geographic points on a continuing basis, how the measured barometric pressure at a particular height above MSL, for example, differs from a standard barometric pressure for that height above MSL.
That difference between measured barometric pressure and a reference barometric pressure, for that geometric height, location and time, can be termed an "atmospheric deviation". It can be shown that if accurate atmospheric deviation data could be made available it would be valuable for many purposes. Weather forecasters, with knowledge of the atmospheric deviation between currently measured and reference pressure values, can analyze atmospheric conditions and forecast developing conditions. Air traffic controllers can apply derived information regarding changes in vertical separation of flight paths resulting from atmospheric pressure changes. Aircraft flight crews can be supplied with altimeter calibration information and data correlating barometric altitude with geometric height. Atmospheric deviation data may also be applied in current calibration of aircraft landing systems, for monitoring developing conditions which may identify wind shear in the vicinity of airports, and for a variety of other civilian, commercial and military purposes. In FIG. 1, curve C is a representation of the use of atmospheric deviation data to calibrate the atmosphere against a standard atmosphere represented by curve S. Thus, with availability of accurate deviation data, it would become possible to provide profile C based upon differences between standard or reference pressure and pressure values based on current barometric measurements, at different altitudes.
While instrumented weather balloons (radiosondes), as well as highly-equipped dedicated aircraft, have been used to gather atmospheric data, these and other existing devices and systems have been expensive, inaccurate and/or used only at a few geographic locations, so that sufficient quantities of current, accurate data have not been available. Thus, it should be noted, that regardless of what forms of theoretical and other systems have been proposed or implemented, a need has continued to exist for a practical, accurate and economical system able to provide a continuing volume of currently updated atmospheric deviation data for dispersed geographical areas of interest, without necessitating specially equipped or dedicated aircraft, development of new forms of equipment or new types of ground installations.
It is, therefore, an object of this invention to provide new and improved systems and methods for determining atmospheric deviations between current atmospheric pressure and reference atmospheric pressure at a point in the atmosphere, as well as for determining accurate geometric height corresponding to a measured barometric altitude, barometric pressure at a point of known geometric height, accurate geometric height of an aircraft, or barometric altimeter calibration for an aircraft while in flight, as may be desired.
It is a further object to provide such systems and methods for making available improved data usable for atmospheric condition analysis, weather forecasting, air traffic control, airport operations, and other uses.
A particular object is to provide such systems and methods which are operable from ground installations through interaction with airborne transponders commonly available in transient aircraft.
Other objects are to provide such systems and methods which avoid cost, accuracy and data availability constraints of prior systems and methods, and which may be implemented simply and economically utilizing existing types of equipment already in use.